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There were several successful attempts to build a convertible ZR-1, most of them by private people. The DR-1 was a GM prototype to test the structural integrity of the ZR-1 chassis when it would be topless. The car was built by American Sunroof Corporation (ASC) for Don Runkle, who was the vice-president of Advanced Engineering Staff, which explains the “DR-1” designation. It was a standard convertible transformed to ZR-1 specs.
The early C4's had their problems, what with the 1984 Cross-Fire Injection and its unusual 4+3 manual transmission. By the late 1980's, the fourth-generation Corvette had evolved into a more desirable automobile, thanks in part to the arrival of the ZR-1. 
Did you know that the earliest ZR1 Corvettes came into existence long before any of these later iterations?  If not, then take a few minutes to acquaint yourself with one the rarest small-block production Corvettes of all time - the 1970-1972 Corvette ZR1. From 1970 to 1972, Chevrolet offered the ZR1 Special Engine Package, which featured a small-block LT1 engine.
The Centennial Edition came finished exclusively in a Carbon Flash Metallic finish with satin-black graphics. Special badging graphics signifying Chevrolet’s racing history, including an image of Louis Chevrolet on the B-pillars, were added as accents to the cars finish. The wheels were also finished in satin black. All models also received red brake calipers. The exterior color scheme of the Centennial Edition Corvette is reinforced inside the car.
This is no ordinary C4 coupe, but a GM Engineering test mule with VIN plate EX4607 proudly displayed in the windscreen, built in 1986 to test all the new-for-1988 features. This actual car must have spent weeks pounding round the General Motors Proving Ground at Milford, Michigan with longer runs on the road, testing all the changes for a year which saw the C4 suspension, steering and brakes vastly improved.